1. Field of the Invention
The invention relates to a process for producing thin slabs with a predetermined convexity of their broad faces in a continuous casting installation, in which an immersion nozzle protrudes into a mold followed by a strand guiding means. The invention further relates to a corresponding apparatus for carrying out the process.
2. Discussion of the Prior Art
German reference DE 41 31 829 C2 discloses a liquid-cooled width-adjustable plate mold for the continuous casting of strands of steel in slab format, in particular for a thickness of the slabs below 100 mm. In the plate mold, the form of the broad-face plates at the strand outlet end of the mold corresponds to the strand format to be produced, the broad-face plates being designed as a planar surface in the adjusting region of the narrow-face plates.
This document does not give any suggestion that the strand format to be produced is also to have a convexity after leaving the mold.
German reference DE 36 27 991 discloses an apparatus for continuously casting flat slabs, in particular a steel slab with a thickness below 80 mm. In this apparatus there is, opposite the larger crowned cross section on the charging side, a cross section on the strand outlet side of the mold which is smaller and identically crowned in the central region, and at least one roller of at least one pair of rollers of the supporting and guiding means following the mold has a caliber adapted to the emerging crowned strand.
The mold form known from this document as well as the form of the supporting and guiding means following the mold are designed in such a way that the mold has in the edge region a form adapted to the strand format. In other words there are already in the mold parallel side wall regions, which continue in the surrounding and guiding rollers of the strand guiding framework.
Germman reference 44 03 0 45 discloses a continuous casting installation for guiding strands of which the broad-face plates are made concave and the concavity is constant from the upper edge of the mold to the outlet of the mold and beyond to the last roller of the strand guiding means. The concave form in this case advantageously runs from the beginning of one narrow-face plate to the beginning of the other, opposite narrow-face plate.
The concave form of the broad-face plates known from this document concerns a relatively complicated form, which is influenced substantially by the flexure of the roller and the wear at the time.
The strand shell in the middle mold-width region, and consequently in the region of the pouring gate is disadvantageously subjected to a constant bending deformation as a consequence of the drawing-off movement until it leaves the region of the pouring gate.
In the documents cited, the graduation of the thickness-reduction steps with respect to the width profile of the strand is not clearly definable with respect to the strand thickness deformation with a liquid crater directly beneath the mold, the so-called cast rolling.
The object of the invention is to provide by simple constructional means a continuous casting apparatus having a mold and strand guiding rollers which reduce the loading on the strand shell and minimize the risk of longitudinal cracks and break-outs.
According to the invention, the broad faces of the mold are largely made up of planar surface parts and the strand guiding rollers have a contour which is made up substantially of straight lines. Right from the inlet of the mold there is provided in the central region a planar surface, which is maintained in the strand guiding direction and, beyond the mouth of the mold, is taken over completely by the central parts of the guiding rollers.
On both sides of this planar central part there are likewise provided planar surfaces in the direction of the narrow faces. These planar surfaces are exactly maintained both in their form and in their inclination from the inlet of the mold up to the end of the strand guiding framework.
Between the planar central surface of the mold and the planar side surfaces arranged on both sides there are provided transitional pieces. The extent of these transitional pieces ends within the mold, with the result that the lower region corresponds to the strand format. In addition, this form allows simple introduction and delivery of the cold strand when starting up the continuous casting installation.
In an advantageous configuration, the central part is shaped with a planar surface in the charging region. The planar central parts of both broad faces of the slab run conically toward each other in the direction of the strand, until within the mold they are guided in parallel, forming a so-called crown, up to the mouth of the mold.
In a further advantageous configuration, the central parts are planar in their surface and disposed in parallel in the charging region and, outside the shadow region of the immersion nozzle in the strand guiding direction, are connected by a connecting part to the central part having the xe2x80x9ccrownxe2x80x9d in the region of the mouth of the mold. The central parts have in this case a form of which the contour lines are parallel to one another and of which the longitudinal extent is designed in the form of an S in the strand conveying direction. The mouth of this S-form respectively goes over tangentially into the neighboring surfaces.
The slab produced in a mold according to the invention has broad faces which are made up of three planar surfaces, the side surfaces being conically shaped and the central surface being shaped with an elevation in comparison with the edge region. This form of slab makes better centering of the slab possible, especially with the strand drawing-off speeds customary nowadays. Uncontrolled movement of the strand in the mold and so-called snaking in the strand guiding framework are avoided as a result.
The outer form of the strand shell of the slab thus produced remains absolutely constant, at least as far as the lowest point of the liquid crater. The only change which the slab undergoes takes place in the direction of its thickness, only the narrow faces being deformed.
The middle mold-width region, to be precise the region designed as a trough, remains unchanged in its planar form until solidifying right through and ensures the most favorable lubricating conditions in the mold. The mold form according to the invention has the effect that the casting powder wets the surface of the strand with an amount which can be reliably predetermined in the region of the greatest susceptibility to longitudinal cracks. Thus, thanks to its planar form, the strand shell is not subjected to any bending stress favoring the occurrence of cracks close to the surface in this middle mold-width region.
The solidifying conditions of the strand shell are especially influenced in the region of the transitional parts and the connecting part by separate channelling of cooling media.
The following strand guiding framework has supporting and guiding rollers, which ensure reliable transporting of the slab still having a crater. According to the invention, various forms of roller are proposed, to be precise complete rollers or else split rollers.
In the case of the split rollers, use is made of simple cylindrical rollers which are adapted to one another according to the shaping of the central or side surfaces of the slab form predetermined by the mold.
Furthermore, it is proposed to divide the rollers in a ratio of 2/3 to 1/3, and to carry out this division alternately. In this case, the 2/3 roller has a contour corresponding to the assignment of the central part to the side part.
Depending on the slab width, in the case of smaller dimensions in particular, use may be made of complete rollers, which have as their contour the negative form of the lower part of the mold.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.